1) Adrenoleukodystrophy (ALD) is a disease of unknown etiology in which there is a significant increase in very long chain fatty acids (mostly 26:0) in tissues. We have studied the effects of medium lipids on hexacosanoate content of cultured normal and ALD skin fibroblasts. It was reduced by 16-30% on replacing growth medium with one containing solvent-extracted serum. Uptake and subsequent release of labeled lignoceric acid (24:0) was similar in ALD and normal cells. Addition of 50 MuM oleate to medium caused reduction in hexacosanoate content in cells. 2) Human skin fibroblasts are capable of rapid desaturation of linoleic acid to fatty acids with 3 and 4 double bonds. Desaturation was greatly increased following incubation of cells in a medium containing lipid-free serum. Sera of patients with Sjogren-Larsson Syndrome (SLS) were previously reported to be deficient in products of linoleate desaturation. Fibroblasts grown from two SLS patients desaturated linoleate at the rate of approximately one-half of control cells. The significance of this finding is presently uncertain.